There has been known an acoustic wave device that includes an Interdigital Transducer (IDT) that excites an acoustic wave and is located on a piezoelectric substrate, and uses a surface acoustic wave propagating through the upper surface of the piezoelectric substrate. The acoustic wave device is light and small, and has a large attenuation to signals outside a predetermined frequency band, being used as a filter and a duplexer of a wireless communication device such as, for example, a mobile phone terminal.
In the acoustic wave device using the surface acoustic wave, a bulk wave propagating through the inside of the piezoelectric substrate is generated in addition to the surface acoustic wave when the IDT excites the surface acoustic wave. When the bulk wave is reflected by the lower surface of the piezoelectric substrate and reaches the upper surface, the characteristics of the acoustic wave device deteriorate. Thus, to inhibit the bulk wave from being reflected by the lower surface of the piezoelectric substrate and reaching the upper surface, the lower surface of the piezoelectric substrate may be made to be uneven as disclosed in Japanese Patent Application Publication No. 2000-278090, or a reforming layer may be formed on the lower surface of the piezoelectric substrate as disclosed in, for example, Japanese Patent Application Publication No. 3-114310.
In addition, there has been known a technique that adjusts vibration frequency by forming a reforming layer inside the piezoelectric substrate as disclosed in, for example, Japanese Patent Application Publication No. 2007-324513. In addition, there has been known a technique that inhibits a bulk wave from reaching one of adjacent resonators from the other by forming a reforming layer in a piezoelectric substrate between the adjacent resonators as disclosed in, for example, Japanese Patent Application Publication No. 2004-336503.
However, when protrusions and recesses or a reforming layer is located on the lower surface of the piezoelectric substrate to inhibit a bulk wave from being reflected by the lower surface of the piezoelectric substrate and reaching the upper surface, the strength of the piezoelectric substrate decreases.